1. Field of the Invention
This invention pertains to the field of secondary and tertiary recovery of oil.
2. Description of the Prior Art
Petroleum in subterranean reservoirs, hereinafter referred to as oil, is often driven to production wells by encroaching water. This encroaching water may come, for example, from a large expanding aquifer. On the other hand water may have been injected into the reservoir to drive oil toward a production well as part of a secondary recovery operation. In either case a large amount of oil is likely to be left behind in the portion of the reservoir encountered by water. The recovery of this oil is complicated by many factors. One factor is the retentive forces tending to keep the oil from moving toward a production well. These factors are viscosity and capillarity. The other factor complicating the production of this oil is the fact that normal production methods in the reservoir where water encroachment has taken place will tend to produce large amounts of water along with the oil. This water presents a problem of disposal in handling which greatly affects the economics of an oil recovery program.
The oil left behind after water encroachment is often produced by injecting a fluid into the reservoir to drive or displace the oil in the reservoir to a production well. This procedure is called secondary recovery in the case where the encroaching water which swept through the reservoir in the first instance was due to natural forces. i.e., aquifer expansion. Where the injection of a fluid to produce oil follows another artificially induced attempt to add energy to the reservoir (water injection, gas injection, in situ combustion, etc.) the injection of fluid is called tertiary recovery. For simplicity, hereinafter all attempts to inject a fluid into a reservoir to displace oil toward a production well will be referred to as secondary recovery techniques regardless of the sequence or number of events prior to the instant recovery program.
In addition to oil production, the water in the reservoir will also be produced in large quantities as discussed above. Also, as mentioned above, the forces of viscosity and capillarity tend to reduce oil production. The retentive forces of viscosity may be removed, for example, by heating the formation to a point where the viscosity of the reservoir fluid becomes equal to or less than the viscosity of the displacing fluid or by increasing the viscosity of the displacing fluid. However, if the displacing fluid is not miscible with the oil the retentive forces of capillarity will not be removed. To remove the retentive forces of capillarity, for example, it is necessary to use a displacing fluid which is miscible with the oil. If the displacing fluid is miscible with the reservoir oil the interface between the oil and displacing fluid will be removed and, therefore, so will the retentive forces of capillarity.
Displacement efficiency is a term referring to amount of oil removed from the portion of the reservoir actually swept by the displacing fluid. Displacement efficiency may be low due to high surface tension at the interface between the displacing fluid and the oil in the reservoir. If this surface tension can be removed the capillary forces will be reduced to zero and the oil may be completely displaced from the portions of the reservoir contacted by the displacement fluid.
Sweep efficiency is a term referring to the percentage of the reservoir actually contacted or swept by the displacing fluid regardless of the amount of oil removed from the swept portion or displacement efficiency referred to above. A major cause of poor sweep efficiency is associated with the fact that the injected displacement fluid generally has a lower viscosity than the oil to be displaced.
If the viscosity of the fluid displacing the reservoir oil to the production wells is lower than the reservoir oil, premature breakthrough of the driving fluid into the production wells will occur. The displacing fluid actually fingers through the reservoir and proceeds to the production well before an adequate portion of the reservoir has been swept. The effects of viscosity on sweep efficiency may be described in terms of the mobility ratio. The mobility ratio is defined by the following equation: ##EQU1## where M = mobility ratio;
u.sub.2, u.sub.1 = viscosity of displacing fluid and displaced fluid (oil), respectively; PA1 K.sub.2, K.sub.1 = permeability of the formation with respect to the displacing fluid and the displaced fluid respectively.
At high mobility ratios the phenomenon commonly known as "fingering" occurs and the displacing fluid does not display a flat front to the reservoir oil but instead rushes ahead at various points in finger-like protrusions which may prematurely break through to the production wells. The oil in areas not touched by the fingers of displacing fluid are usually left unrecovered in pockets in the reservoir. These pockets are isolated and are likely to be lost forever. The preceding equation shows that the mobility ratio and the degree of fingering is directly proportional to the ratio of the displaced fluid viscosity to the displacing fluid viscosity u.sub.1 /u.sub.2. Since most displacing fluids are less viscous than the displaced fluid (oil) the mobility ratio will usually be quite high and a poor aerial sweep efficiency will occur because of fingering.
This invention provides a method for increasing the amount of oil production relative to water production by solving the problems of viscosity and capillarity inhibiting oil production or providing at the same time a method of retarding water production.